docs: def unsubscribe(self, subscriber: 'Subscriber') -> None:
          """ Unsubscribe the given subscriber
  
          :param subscriber: subscriber to unsubscribe
          :raises SubscriptionError: if subscriber is not subscribed (anymore)
          """
          # here is a special implementation which is replacing the more
          # obvious one: self._subscriptions.remove(subscriber) - this will not
          # work because list.remove(x) is doing comparision for equality.
          # Applied to publishers this will return another publisher instead of
          # a boolean result
          for i, _s in enumerate(self._subscriptions):
              if _s is subscriber:
                  self._subscriptions.pop(i)
                  return
          raise SubscriptionError('Subscriber is not registered')